A method and a device of this type are already known.
It is known that, when the frequency of the microwave signal applied to the atomic resonator is equal to f.sub.o .+-.f.sub.m (f.sub.o being the frequency of the atomic resonance), the response of the resonator exhibits extremums when the amplitude of the microwave signal is varied, thereby permitting to interlock the amplitude of the microwave signal in follow-up relationship with a value which only depends on the properties of the resonator, which are nearly insensitive to ambient disturbances such for example as the temperature. It is the most marked extremum, the first maximum in this case, which is advantageously used.
Now in the case of a caesium jet clock, the atomic response is not symmetrical with respect to a vertical line passing through the maximum when the abscissa is proportional to the amplitude of the signal. In this case, the maximum may be obtained with a good accuracy only if the depth of the modulation of the microwave amplitude is small. The error signal which permits to control the value of the amplitude is then obtained with a small signal-to-noise ratio, thereby imposing a permanent modulation of the amplitude.
Therefore the known methods and devices, of the kind stated hereinabove, carry out a not very marked modulation of the amplitude of the microwave signal which is applied permanently. This technical process exhibits the inconvenience that the amplitude modulation and the frequency modulation are intermingled with each other, thereby providing very complex if not too complex conditions in particular in the case of a caesium jet clock with an optical pumping.